Why are the two terms labeled as 1 and 2 connected by a minus sign?

Why are the two terms labeled as 1 and 2 connected by a minus sign?

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a. Because an increase in the population of species A has a negative effect on the growth of species B
b. Because an increase in the population of species A has a positive effect on the growth of species B
c. Because an increase in the population of species B has a negative effect on the growth of species A
d. Because an increase in the population of species B has a positive effect on the growth of species A